1. Field of the Invention
The present invention relates to a method for detecting a quantity of laser scanning positional deviation on a photosensitive body, a method for correcting scanning positional deviation, and a laser color image forming apparatus using a plurality of laser beams, which is provided with such methods.
2. Description of the Related Art
Heretofore, in a color image forming apparatus using a plurality of laser beams and an electrophotographic photosensitive body, a difference in distance has been generated by an attaching error, temperature fluctuation, and the like between a laser scanner unit and a photosensitive belt among colors, causing variance in laser beam scanning velocities on the photosensitive belt. Consequently, color shifting has occurred in a formed color image. In a monochromatic image forming apparatus using laser beams and an electrophotographic photosensitive body, as methods for solving such a problem of fluctuation in the laser beam scanning velocity, Japanese Patent Laid-Open No. 63 (1988)-175817, Patent Laid-Open No. 62 (1987)-253115, Patent Laid-Open No. 3 (1991)-110512, and Patent Laid-Open No. 61 (1986)-162023, and the like, discloses technologies. A basic principle of the methods provided by any of these technologies is as follows. That is, as shown in FIG. 9, photodetection sensors are provided, one each, in the start and finish end portions of a laser beam, a scanning velocity is obtained from the time of laser beam scanning between the photodetection sensors, the scanning velocity is fed back to a pixel clock frequency, and then the pixel clock frequency is corrected.
However, there are two significant drawbacks when the above methods are applied to a color printer using a plurality of laser beams.
First, the attaching positions of the sensors must be completely identical among colors. Such setting is very difficult, and thus impractical. Second, the methods are advantageous only when the scanning velocity is uniformly shifted without any dependence on a scanning position on the photosensitive body, in other words, only when a positional relation between a laser optical system and the photosensitive body is shifted in parallel to a scanning direction. The methods are ineffective for a positional change which changes the velocity depending on a scanning position, or the like.
Because of such drawbacks, the color shifting inevitably occurs in an image of each color formed on the photosensitive body.
With regard to a method for detecting the quantity of printing positional deviation in an electrophotographic LED color printer, a technology is disclosed in Japanese Patent Laid-Open No. 11 (1999)-157134. According to the method provided by this technology, a laser device, a condenser optical system, and an image-forming optical system, which are all dedicated to the detection, are required for detecting a printing positional deviation detection pattern of each color developed on the photosensitive body. Thus, costs are inevitably high. Further, to increase detecting resolution, a condensing spot is stopped down to about several tens of xcexcm. This necessitates an adjustment also in a detecting system, causing temperature fluctuation, fluctuation with time, and other problems in the detecting system itself.
As regards a color image forming apparatus using a plurality of laser beams and an electrophotographic photosensitive body, the object of the invention is to provide a method and an apparatus for solving the problem that a difference in distance has been generated by an attaching error, temperature fluctuation, and the like between a laser scanner unit and a photosensitive belt among colors, causing variance in laser beam scanning velocities on the photosensitive belt and consequently, color shifting has occurred in a formed color image.
Regarding a laser color image forming method for developing a latent image formed on an electrophotographic photosensitive belt by laser scanning having a fixed scanning frequency and a pixel forming clock frequency within one scanning by toner absorption, and then transferring the developed image to a printing medium, that is, a paper sheet, and in a method using a plurality of laser beams and a plurality of toner colors, a method of the invention is provided for detecting the quantity of laser scanning positional deviation on the photosensitive belt.
This method comprises the steps of:
setting a plurality of zonal patterns formed by toner development on the photosensitive belt for optional one among a plurality of colors and arrayed on a fixed cycle as image patterns for detection of a printing positional deviation of a reference color, setting image patterns having shapes similar to those of the image patterns for the detection of the printing positional deviation of the reference color and designed for the detection of the printing positional deviation of other colors as image patterns for the detection of the printing positional deviation of a color to be measured, spatially shifting the image patterns for the detection of the printing positional deviation of the color to be measured from the image patterns for the detection of the printing positional deviation of the reference color, and varying the quantity of spatial shifting for each of the plurality of the image patterns for the detection of the printing positional deviation of the reference color, thus forming the image patterns in a superposed state on the photosensitive belt;
measuring reflection densities of the plurality of image patterns formed in the superposed state for the detection of the printing positional deviation;
detecting a group having said minimum reflection densities among the plurality of the image patterns formed in the superposed state for the detection of the printing positional deviation; and
detecting the quantity of the deviation between a laser scanning position of the reference color and a laser scanning position of the color to be measured based on the quantity of shifting given when the detected group of image patterns for the detection of the printing positional deviation is formed by being superposed on the color to be measured.
In this case, the image patterns for the detection of printing positional deviation of the reference color and the color to be measured are formed in three places including a scanning start end portion, a scanning central portion and a scanning finish end portion of a laser scanning direction on the photosensitive belt.
Moreover, the image patterns for the detection of printing positional deviation of the reference color and the color to be measured are constituted of image patterns for detecting printing positional deviation in the laser scanning direction on the photosensitive body, and for detecting printing positional deviation in a sub-scanning direction orthogonal to the laser scanning direction.
Furthermore, the number of other colors is three, and the quantities of deviation with respect to a laser scanning position of the reference color are detected for each of the three colors.
In addition, the three colors are all developed by a toner color similar to that for the reference color.
Furthermore, the measured reflection densities of the image patterns for the detection of printing positional deviation are obtained from an irradiating light used to irradiate the plurality of zonal patterns arrayed on the fixed cycle constituting the image patterns for the detection of printing positional deviation.